1. Key Difference 1: Animation Capabilities Anime.js is primarily a JavaScript animation library that focuses on providing smooth and visually appealing animations for web interfaces. It allows developers to create animations with a wide range of properties such as colors, transforms, and even SVG morphing. On the other hand, Three.js is a JavaScript library built specifically for creating and rendering 3D graphics in a web browser. It offers a comprehensive set of tools for creating complex 3D scenes, with features like lighting, shading, and geometry manipulation.

2. Key Difference 2: Purpose and Usage Anime.js is designed to be used for animating various elements on a webpage, including text, images, and DOM elements. It provides a simple and intuitive API, making it easy to apply animations to HTML elements with just a few lines of code. On the contrary, Three.js is focused on creating immersive 3D experiences on the web. It is typically used to build interactive 3D games, virtual reality simulations, or architectural visualizations that require rendering complex 3D objects and environments.

3. Key Difference 3: Learning Curve Anime.js has a relatively low learning curve, ideal for developers who want a quick and easy way to add animations to their websites. Its syntax is straightforward and follows a declarative approach, allowing developers to specify the desired animation properties directly. In contrast, Three.js has a steeper learning curve due to its focus on 3D graphics programming. It requires a solid understanding of concepts such as geometry, shaders, textures, and matrix transformations, making it more suitable for developers with a background in computer graphics.

4. Key Difference 4: Browser Compatibility Anime.js is designed to work seamlessly across different browsers, including modern and older versions. It leverages CSS animations and transforms, which are supported by a wide range of browsers, ensuring consistent animations across different platforms. On the other hand, Three.js relies heavily on WebGL, a web standard for rendering 3D graphics, which may not be available or supported in older or less capable browsers. This limits the compatibility of Three.js-based projects to more modern and capable browsers.

5. Key Difference 5: Community and Ecosystem Anime.js has a relatively smaller community compared to Three.js, but it still has an active and growing user base. It has a dedicated GitHub repository with regular updates and bug fixes. However, due to its focus on animations, the ecosystem around Anime.js might not be as extensive or diverse as that of Three.js. Three.js, on the other hand, has a large and vibrant community with a wide range of plugins, extensions, and resources available. It also benefits from its association with WebGL, which has a strong developer community and a wealth of resources.

6. Key Difference 6: Extensibility and Customizability Anime.js provides a solid foundation for creating animations, but its primary focus is on ease of use and simplicity, limiting the extensibility and customizability of its core features. While it does offer certain advanced features like custom easing functions and timeline control, it may not be as suitable for highly complex or specialized animation requirements. On the contrary, Three.js is highly extensible and customizable, allowing developers to create their own shaders, materials, and controls. This flexibility makes it an ideal choice for developers who want full control over every aspect of their 3D graphics.

In Summary, Anime.js is a JavaScript animation library that simplifies the process of adding web animations, while Three.js is a comprehensive JavaScript library for creating and rendering 3D graphics on the web with extensive community support. The key differences include animation capabilities, purpose and usage, learning curve, browser compatibility, community and ecosystem, and extensibility and customizability.